The present invention relates to an improved process for the production of linear monoolefins by the catalytic dehydrogenation of the corresponding normal paraffins. More specifically, the invention relates to an improved process for the production of linear monoolefins containing from about 9 to about 15 carbon atoms per molecule. Such monoolefins, referred to herein as detergent range olefins, are particularly useful as reactive intermediates in the production of biodegradable detergents. This invention relates to specific combinations of reaction conditions different from those suggested by the prior art which have surprisingly been discovered to be quite advantageous for use with platinum catalysts in the production of detergent range olefins.
It is known that dehydrogenation processes employing noble metal catalysts are effective for the production of detergent range olefins, with catalysts comprising platinum being particularly effective. Rather broad ranges of reaction conditions which can be used in such processes are also known in the art. However, the production efficiency can vary widely depending upon the properties of the catalyst and the particular combination of reaction conditions selected.
It is known that in the catalytic dehydrogenation of normal paraffins, the percentage conversion to monoolefins in a single pass is subject to a severe equilibrium limitation. While the limiting conversion can vary considerably with the reaction conditions, the actual percentage of monoolefin in the products is typically not greater than about twenty weight percent of the total hydrocarbons present. It is also well known that the formation of monoolefins in such processes is accompanied by the formation of various less desirable byproducts including diolefins, aromatics, and hydrocarbons with carbon numbers below the detergent range which are formed by cracking reactions. As used herein, the term "conversion" means the percentage of the normal paraffins in the feed which are converted in a single pass to species other than normal paraffins within the same carbon number range, and the term "selectivity" means the percentage of the converted normal paraffins which are converted to monoolefins within the same carbon number range. In cases in which the feed contains species other than normal paraffins within the desired carbon number range, these species are ignored in the calculation of conversion and selectivity. In general, higher conversion and higher selectivity are advantageous, but an increase in conversion tends to lower selectivity.
Another well known problem encountered in the production of detergent range olefins by catalytic dehydrogenation of normal paraffins is the loss of catalyst activity during its use. The catalyst can lose activity as a result of strong catalyst poisons such as sulfur in the feed, and such activity loss is generally controlled by controlling feed purity. But even when the feed contains extremely low levels of such poisons, the catalyst tends to deactivate at a significant rate due to the formation of coke on the catalyst surfaces. The rate of coke formation can vary widely depending upon the combination of reaction conditions selected. In general, a lower rate of coke formation is advantageous since this reduces various costs associated with catalyst regeneration or replacement and facilitates the maintenance of both conversion and reaction conditions within optimum ranges for extended periods of operation.
One method used in prior art paraffin dehydrogenation processes to reduce catalyst deactivation due to coke formation is to mix varying amounts of hydrogen with the vaporized paraffin feed prior to its introduction into the catalytic reaction zone. It is taught in U.S. Pat. No. 4,343,724 for example that such hydrogen serves a "dual-function" in both diluting the paraffin and "suppressing the formation of hydrogen deficient, carbonaceous deposits on the catalyst composite." In many cases, the amount of hydrogen used in patent examples has been extremely large, for example 4-8 moles of hydrogen per mole of hydrocarbon. Severe disadvantages accompany such large additions of hydrogen, including an adverse effect upon the equilibrium for monoolefin formation, increased size of most portions of the processing equipment for a given production rate, and increased energy and maintenance costs associated with the recovery, recompression, and recycle of hydrogen. Thus, it is greatly advantageous to reduce the hydrogen to hydrocarbon mole ratio (H2:HC) used in the process. However, the prior art suggests that a H2:HC ratio of at least about 2.0 is required to obtain favorable results.
Prior art teaching related to H2:HC ratios and reaction temperatures for use in platinum catalyzed dehydrogenation of detergent range normal paraffins is illustrated in Table I. Unless otherwise noted, the H2:HC ratios apply to the reactor feed mixture, and the temperatures are reactor inlet temperatures for use with a substantially adiabatic reactor. The broad ranges generally apply to a carbon number range wider than C.sub.9 -C.sub.15. Preferred ranges may also apply to a wider carbon number range, but if more than one preferred range was mentioned, that most applicable to detergent range feedstocks, about C.sub.9 to about C.sub.15, was used in the table. All examples applicable to detergent range feedstocks were included under the heading of examples.
In Table I, the lowest H2:HC ratio given in any example of an operable process is 2.0. In U.S. Pat. No. 3,274,287 there were a few examples with no added hydrogen, but these were included to illustrate the failure of the process under such conditions. Since the examples in any given patent should generally include the best known mode of operation, such teaching leads one to expect that the H2:HC ratio must be at least 2.0 to obtain satisfactory results. Similarly, these teachings lead one to expect that the inlet temperature for a single stage adiabatic reactor would best be selected from a range of about 427.degree. C. to about 575.degree. C.
TABLE I __________________________________________________________________________ Prior Art Choices of H2:HC Ratio and Temperature H2:HC Ratio Ranges Temperature Ranges, .degree.C. U.S. Pat. No. Broadest Preferred Examples Broadest Preferred Examples __________________________________________________________________________ 4,827,072 0.1-20 1-10 4 200-1000 525-700 495 4,677,237 0.1-40 1-10 4 400-900 -- 495 4,608,360 0.1-40 1-10 4 400-900 -- 495 4,595,673 0.1-40 1-10 4 400-900 -- 495 4,551,575 1-40 1.5-10 6 400-900 -- 485 4,486,547 1-40 1.5-10 6 400-900 -- 485 4,396,540 1-20 1.5-10 4-5 371-704 427-510 443-454 4,343,724 1-20 1.5-10 4-5 371-704 427-510 443-454 4,341,664 1-20 1.5-10 4-5 371-704 427-510 443-454 4,312,792 1-20 1.5-10 4-5 371-704 427-510 443-454 4,268,706 1-20 1.5-10 4-5 371-649 427- 510 443-454 4,227,026 1-20 1.5-10 4-5 371-649 427-510 443-454 4,216,346 1-20 1.5-10 4-5 371-649 427-510 443-454 4,177,218 1-20 1.5-10 8 375-650 375-550 465 4,172,853 1-20 1.5-10 8 371-649 427-510 443-460 4,136,127 1-20 1.5-10 5 371-649 427-510 449-460 4,133,842 &lt;15 -- 7.5 399-566 427-510 460 4,125,565 1-20 1.5-10 4-5 371-649 427-510 443-454 4,070,413 1-20 -- -- 400-700 450-550 -- 4,048,245 1-20 1.5-10 5 371-649 427-510 438-454 3,998,900 1-20 1.5-10 8 371-677 427-510 449-466 3,920,615 0.1-50 1-5 2-8 400-650 420-520* 430-450* 3,907,921 -- -- -- 430-540 460-485 -- 3,825,612 1-20 1.5-10 8 371-649 427-510 449-466 3,767,594 1-10 -- 8 399-704 -- 460-475 3,761,531 1-20 5-15 8 371-649 427-510 443-475 3,662,018 -- -- 8 427-510 -- 454-460 3,649,566 1-20 1.5-10 8 371-677 427-510 449-466 3,647,719 1-20 1.5-10 8 371-677 427-510 449-466 3,632,662 -- -- 2 -- -- 452* 3,585,253 0.1-50 1-5 2 400-650 420-520* 450* 3,576,766 1-20 1.5-10 8 371-677 427-510 449-466 3,527,836 0.1-30 2-10 4.1 400-650 -- 440 3,458,592 0.5-15 -- 5.5-7.8 427-510 454-488* 454-460* 3,448,165 &lt;15:1 &lt;10:1 2-8 400-700 430-530 427-575 3,315,008 0.1-5 1-3 2 400-650 520-520* 435-440* 3,315,007 0.1-5 1-3 2 400-650 520-520* 440* 3,293,319 1-10 -- 4-8.8 400-700 -- 430-460 3,274,287 0.5.5 1-3 2.sup.+ 400-500 420-480* 420-440* __________________________________________________________________________ *Cited studies used an isothermal reactor .sup.+ Also includes examples of nonoperability at H2:HC = 0.